1. Field of the Invention
The present invention relates to a direct fuel injection-type spark-ignition internal combustion engine.
2. Description of the Related Art
In a direct fuel injection-type spark-ignition internal combustion engine having a fuel injector for injecting fuel directly into a cylinder, fuel is injected into a cavity formed in the top surface of a piston in the last stage of the compression stroke, is vaporized by utilizing the heat of the high temperature air in the cylinder and of the piston, and is led to the vicinity of the spark plug to form a combustible mixture that can be favorably ignited in the vicinity of only the spark plug at the ignition timing. Thus, stratified charge combustion which can burn a total lean mixture in the cylinder is realized.
In general, as a direct fuel injection-type spark-ignition internal combustion engine disclosed in Japanese Unexamined Patent Publication (Kokai) No. 9-158736, the fuel injector is arranged in the upper periphery of the cylinder on the side of the intake port and the spark plug is arranged nearly at the center in the upper part of the cylinder. The cavity formed in the top surface of the piston has a bottom wall with which the fuel injected from the fuel injector will come into collision and a spark plug side wall. Due to the spark plug side wall, fuel proceeding along the bottom wall after collision is deflected to the vicinity of the spark plug.
In the above art, the fuel injector injects fuel in the shape of a flat fan having a relatively small thickness, and fuel proceeding on the bottom wall of the cavity spreads in the direction of width and is easily vaporized by absorbing heat from a wide area of the bottom wall. The spark plug side wall of the cavity has an arcuate shape in a plan view. Therefore, in the central part of fuel in the direction of width thereof, a velocity vector proceeding on the bottom wall is changed into an upward velocity vector by the spark plug side wall, and thus it is directed toward the vicinity of the spark plug. Further, both side parts of fuel in the direction of width thereof collide with the spark plug side wall at an acute angle, whereby a velocity vector proceeding on the bottom wall is converted by the spark plug side wall into a resultant velocity vector of the upward velocity vector and the velocity vector in the central direction, and thus they are directed toward the vicinity of the spark plug. Thus, fuel favorably vaporized and spreading in the direction of width is expected to collect near the spark plug to form a favorable combustible mixture near the spark plug.
Undoubtedly, fuel can be easily vaporized by being injected in the shape of a fan so as to spread in the direction of width as it proceeds on the bottom wall of the cavity. However, on the other hand, fuel in the shape of a fan tends to be spread excessively in the direction of width on the bottom wall of the cavity, particularly, both side parts of the fuel tend to be excessively dispersed.
As described above, each part of fuel in the direction of width thereof is directed to the vicinity of the spark plug by the spark plug side wall of the cavity. However, it is difficult to correctly direct each part of fuel to the same position. In practice, the mixtures formed by all the parts of the fuel are joined together near the spark plug. In order to reliably ignite the joined mixture, therefore, the mixture formed by each part of the fuel must be a combustible mixture that can be favorably ignited.
However, when the both side parts of the fuel are excessively dispersed on the bottom wall of the cavity, the mixture formed by these parts of the fuel becomes very lean and does not favorably burn, making it difficult to produce the engine output as intended and, besides, unburned fuel is emitted in increased amounts.